Spectral Characteristic and Colour Change Mechanism of Colour-Changing Garnet

Colour-changing garnets are characterized by colour-changing effect.Typically, this type of garnet is composed of pyrope, spessartite, or spessartite with trace elements like chromium (Cr) and vanadium (V). This study investigated five groups of colour-changing garnets exhibiting different tones: dark green, grayish blue, blackish gray, yellowish green, and brownish gray. To understand their colouration mechanisms and the cause of discolouration, our study applies a variety of gemmological techniques, including laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), ultraviolet visible spectrophotometer (UV-Vis) and Fourier transform infrared spectrometer (FTIR). The results demonstrated that the samples were spessartite rich in Mn²⁺, Fe²⁺, V³⁺, Cr³⁺, with minor components of grossular and almandine garnet.Analysis of UV-Visible absorption spectrum and chemical composition indicated that Mn²⁺, Fe²⁺, V³⁺, and Cr³⁺ caused the colour and discolouration effects in colour-changing garnets. The broad absorption band centered at 576 nm was associated with Cr³⁺ and V³⁺spin-allowed d-d orbital transitions, specially ⁴A_2g→⁴T_2g and ³T_1g→³T_2g, respectively. Additionally, the 422 nm absorption band in 400-460 nm was assigned to the Mn²⁺ d-d orbital spin-forbidden ⁶A_1g→⁴E_g transition, while the 457 nm absorption peak corresponded to the Fe²⁺ d-d orbital spin-forbidden ⁵T_2g→⁵E_g transition.When the absorption intensity in the 400-460 nm range was high, Mn²⁺ and Fe²⁺ mainly contributed to the colour, resulting in yellow colour under fluorescent lamps. Conversely, with lower absorption intensity, yellow colour dominated by Cr³⁺ and V³⁺ were absorbed, leading to blue-green hues under fluorescent lamps. The intricate interplay of absorption bands, particularly the 576 nm and 400-460 nm bands, resulted in distinct transmittance regions at 506 nm (green) and 660 nm (red) beyond. The observed colour-changing and discolouration effects in colour-changing garnets are a manifestation of the selective transmission through different light regions, influenced by the varying compositions of light sources.